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If there's one thing you'd think we don't need more of, it's jellyfish. They swarm through every ocean in the world, sting anything that comes near them and take over local ecosystems when fish stocks drop. What's more  and let's be honest  any creature that's been around for 500 million years and still hasn't figured out how to grow a brain was never going to amount to much anyway.

But as a paper published in Nature Biotechnology reports, a team of bioengineers at Caltech and Harvard  places they grow very big brains indeed  has announced that it has built what is essentially a living, swimming artificial jellyfish. And for folks who think the team could at least have picked a species that needs a little help  pandas, anyone?  the researchers stress that what they learn in the lab could eventually be put to work saving human lives.

Jellyfish serve as a model for bioengineers for the same reason yeast were once so valuable to geneticists: they're simple to deconstruct. A jellyfish is little more than a pulsating bell, a tassel of trailing tentacles and a single digestive opening through which it both eats and excretes  as regrettable an example of economy of design as ever was. But it's the bell that interested the investigators most. All muscles  including those that make up the heart, the intestinal system and other organs  operate on a common principle of individual cells contracting in an orderly sequence. The better scientists can understand that phenomenon, the better they can repair damaged tissue or build replacement parts.

"It occurred to me in 2007 that we might have failed to understand the fundamental laws of muscular pumps," says bioengineer Kevin Kit Parker of Harvard, a co-leader of the study. "I started looking at marine organisms that pump to survive. Then I saw a jellyfish at the New England Aquarium, and I immediately noted both similarities and differences between how the jellyfish pumps and the human heart."

Parker reached out to John Dabiri, a Caltech professor of aeronautics and bioengineering, and the two of them, along with Caltech doctoral student Janna Nawroth, spent several years mapping the muscle fibers in the jellyfish bell. Over time, they were able to develop a sort of schematic of the tissue that could, in theory at least, serve as an instruction manual for building their version of the critter.

They began that assembly work with a thin polymer membrane that closely replicates the flexibility of real jellyfish tissue and cut it to the size and vaguely eight-armed shape of a small jellyfish. They then harvested cardiac cells from the hearts of rats. Scattering the individually pulsating cells over the polymer could cause it to move after a fashion, but the motion would be uncoordinated and random. Instead, the researchers laid down a road map of protein along the membrane, following the jellyfish blueprint they had drawn up. Rat cells applied to the polymer ought to follow those nutrient routes and grow in just the right arrangement.

That, as it turned out, is exactly what happened. And when the investigators immersed their invention in a tank of electrically conductive fluid and hit it with a fluctuating current, the polymer jellyfish behaved just like the genuine article  which is to say, it swam, and with proper jellyfish motion too.

"I'm pleasantly surprised at how close we are to matching the natural biologic performance," says Dabiri.

Practical applications of the work are still a long way off, but a good first target would be to use the technology to develop a living pacemaker for human hearts  one that doesn't have to be replaced or maintained, never mind recharged. That could lead to the construction of more complex pumping or contracting organs, including the bladder. Eventually, such work could become almost routine.

"As engineers," says Parker, "we are very comfortable with building things out of steel, copper, concrete. I think of cells as another kind of building substrate, but we need rigorous, quantitative design specs to move tissue engineering from arts and crafts to a reproducible type of engineering."

In the short term, the researchers would like to improve on their jellyfish so that it can move on its own, without needing to be prodded by a current. They even imagine giving it the capacity to gather its own food  though they haven't quite figured out a way to do that yet. But there are no plans now  or ever  to give the thing a brain. Its cousins in the wild already had their chance at that.